Franklin County's population has grown faster by percentage than any other county in the state since the 2000 census, according to new estimates by the Washington State Office of Financial Management.

Pasco and Kennewick also are among the state's 10 fastest-growing cities based on numbers of residents, and Richland ranked 16th out of 279 cities and towns.

State population growth overall is slowing, according to the estimates, which are prepared annually. The state's population is estimated to have increased by 1.2 percent to 6,668,200 in the last year compared with a peak growth in recent years of 1.9 percent in 2006.

Migration into the state is driven by job opportunities, and Washington's economy remains more attractive than California or Oregon, the states that traditionally have had the most residents moving to Washington. But population gains statewide due to migration still have dropped from 81,000 in 2006 to 39,000 in 2009, said Theresa Lowe, Washington's chief demographer, in a statement.

"Many job seekers are finding it difficult to sell their homes or to relocate to accept employment at the price of paying two mortgages for an extended period," she said.

Franklin County's population has grown 47.3 percent since the 2000 census, making it the fastest growing based on percentage change, the new estimates said. It's estimated to have 72,700 residents, up from 49,347 in 2000. Births accounted for more new residents than people moving into the county.

Benton County has grown from 142,475 people in 2000 to an estimated 169,300, with people moving into the county slightly outpacing births. Benton County ranked fifth in the state.

Pasco is the state's fourth-fastest-growing city based on number of increased residents. Its population increased from 32,066 in 2000 to a current estimate of 54,490. That included 1,769 new residents who live on property annexed into the city.

The fastest-growing city was Seattle, followed by Renton and Auburn, which both had at least half of their population increase attributed to annexation.

Kennewick ranked eighth in the state, with its population growing from 54,751 to an estimated 67,180. More than a third of the increase was based on annexation.

Richland grew from 38,708 residents to an estimated 47,410 people with just 35 new residents from annexation. West Richland ranked 39th, with its population increasing by 3,285 people since 2000 to an estimated 11,670.